Deck Board Calculator

The short answer

Decking boards at the common 5/4 by 6 section are 5.5 inches wide, so a 100 square foot deck needs about 218 linear feet of board — 100 times 12 divided by 5.5 — and about 240 with 10% for cuts and defects. Joists typically run at 16 inches on centre, and a 12 by 12 deck needs framing for both directions plus beams and posts at the bearing points.

What it depends on

Board width and the gap you leave between boards change the coverage: a 1/8-inch gap on 5.5-inch boards adds about 2% to the linear footage. Diagonal layouts add roughly 15% for waste. Framing depends on the span and the load, not on the area, which is why two decks of the same size can need different joist sizes and post spacing.

Where this stops being useful

This is a material estimate. Deck structures are engineered items: joist and beam sizing, post spacing, footing depth below frost line, ledger attachment and guardrail height are all governed by code and by local conditions, and permits are usually required. A structural engineer or licensed contractor should design anything you will stand on. General information and planning only, not engineering, structural or code advice.

Decking is measured by linear footage of board, and the spacing between boards changes that figure. Boards laid on a quarter-inch centre-to-centre spacing need more length than boards laid tight, and the spacing is determined by the board's actual width as delivered, not the nominal width on the spec sheet — which is a persistent and costly surprise. Joist spacing also matters structurally: most residential decking requires support every sixteen inches, and wider spans need a header or extra support.

A deck that has been thought through as a drawing goes together far more smoothly than one built from a linear footage figure. The layout determines the joist positions, which determine the footings, which determine the post locations, and the whole sequence depends on that first sketch.

Framing material, decking, posts and footings are four separate order quantities driven by the same layout. Draw the frame on paper first with every joist position marked, then measure what each piece has to be, and only then order. Footings and posts are determined by the joist span and the soil, not by the deck size alone. On a low deck attached to the house, ledger flashing and a proper connection to the structure are what keep water out of the house — this is the detail that fails first and costs the most to fix.

A deck estimate is really two estimates: the decking surface, which runs one way, and the structure beneath, which runs across it. Getting the first right means thinking in rows of boards rather than in area, because boards are fixed in width and the gap between them is part of the design, not wasted space. The number of rows is the deck width converted to inches, divided by the width of one board plus one gap, rounded up. That gives how many boards you need to span the width, each cut to the length of the deck. Multiplying rows by length gives the linear feet of decking, and dividing by the length of the boards you plan to buy gives the piece count. Wider boards mean fewer joints and faster laying but more visible movement with moisture; narrower boards look busier and are less prone to cupping. The gap is a design choice with consequences. A quarter-inch gap lets water drain and debris pass and allows the boards to swell without buckling. Too narrow and the deck traps water and clogs; too wide and it becomes uncomfortable underfoot and lets small objects through. The gap also reduces the surface area you cover per board, which is why it must be added to the board width when counting rows rather than ignored. This calculator returns rows, boards, total linear feet and the number of joists at your chosen spacing, with a waste allowance for end cuts and defects. It assumes a rectangular deck framed with joists running across the shorter dimension, and does not include posts, beams, stairs or railings, which are estimated separately. Check the span tables for your lumber species before fixing joist spacing.

This calculator returns estimates for planning and general information only. It is not engineering, architectural, structural, electrical or code advice, and it cannot replace a qualified professional who has seen the site. Material quantities depend on the actual dimensions, the product you buy and local building codes, so confirm the figures before ordering materials or starting work, and always follow local regulations.